Dirt traps are an essential part of any gas system installation but there are several parameters the dirt trap must feature, not like the one below.
If there is any debris or particles present in the Fixed Gaseous Fire Extinguishing Systems pipework, it will be pushed through the pipe on system discharge and can collect in the nozzles at the end of each pipe run blocking the nozzle(s) and adversely affecting the uniform distribution of the gas agent. This may also produce contamination of the protected area/asset.
SOURCES OF CONTAMINATION
Particulate contamination within the pipework can arise from:
- Swarf from thread cutting, welding slag, detachment of burrs etc during the site fabrication processes
- Accidental entrapment of loose objects during installation.
- Particulate contamination into open nozzles via airborne particulate matter
- Surface corrosion forming over time on the inside bore of the pipe, particularly where the pipework passes through outdoor or unheated areas.
CONTAMINATION REMOVAL
The standards require that pipework be blown through with compressed gas before the nozzles are fitted, to remove any particulate matter. The pressure used and duration of the blow through are not specified, but the pressure and hence flow rate will be much less than that of a discharge. It is likely that particles that may be held by surface tension of any remnants of oil or cutting fluid may not be entirely removed by the blow through. Consequently, the blow through may only be effective in removing small, loose particles, whereas a full discharge can dislodge stuck particles and transport larger ones. Ideally internal swabbing of each pipe section should be done once the threads are cut, before installation, however such practice does not negate the blow through requirement.
DIRT TRAP CONSTRUCTION
The dirt traps required by the standards consist of a tee fitted with a nipple and a blanking cap at least 50 mm long and is fitted to each end of pipe run nozzle. The nozzle should be fitted on the branch of the tee with the dirt trap on the through outlet of the tee. This configuration relies on any particles present having a certain mass and therefore momentum to carry them past the last nozzle position and into the dirt trap.
Note: the through outlet of the T should be capped and not “plugged”
Consideration should be given to using dirt traps fabricated from a short piece of capped pipe of 50 mm or longer, for example 3 times the pipe diameter.
STANDPIPES
Where standpipes are used to provide a single nozzle per container, with a bottom entry nozzle the design may not lend itself to the specified dirt trap design.
it is essential that the standpipe used is ether made under factory cleanliness conditions, or where site fabricated, the completed pipe assembly is thoroughly internally cleaned or swabbed prior to installation.
